The amphibian chytrid fungus Batrachochytrium dendrobatidis was blamed for the decimation and extinctions of frog species around the world in the late 1990s.

But now Retallick and team have shown that at least some frog species have happily lived with the fungus for years.

The researchers studied a frog population, in an east-central Queensland national park, which suffered particularly catastrophic losses in the late 1990s, with one species, the Eungella Gastric-Brooding Frog, becoming extinct.

They studied samples (preserved toe clippings) taken from frogs which had been captured and then re-released to park between 1994 and 1998, before the fungus was identified as a problem.

Samples from the six species of frogs were found to have the fungus. Even more surprising was that subsequent recapture showed that some frogs with the fungus had survived.

The researchers found that on average 15% of the frogs that they examined had the fungus, a higher percentage in summer.

Researcher Associate Professor Hamish McCallum from the University of Queensland, said the team had not expected to find chytrid fungus in the toe clippings, nor to find it existing in frogs today.

"I was surprised to find the disease was still there at substantial levels of prevalence," said McCallum. "What was more surprising was that this didn't seem to affect survival rate."

Two theories

McCallum said there were now two theories on the table about the role of the fungus in the decline of frogs.

One theory is that the fungus never caused the decline in the first place.

"When they declined, the frogs simply disappeared. One day they were there, the next time we looked for them they were gone," he said. "There was never any smoking gun that proved it was the fungus."

"If it wasn't [the fungus], then we still don't know what killed them."

He said a second hypothesis was that less deadly strains of the fungus had evolved and the frogs had developed immunity to the disease.

McCallum said it could be similar to the case of the lessening effectiveness of myxomatosis virus introduced to eradicate rabbits in Australia. Over time, the rabbits evolved resistance to the virus and the virus itself became less virulent.

"Animals evolve resistance to lethal pathogens going though a population very rapidly," McCallum said. At the same time the pathogen evolves towards a lower virulence because it doesn't spread as effectively, he said.

After five years the virus strain that originally killed 99% of rabbits was only 77% effective, he said, although he added that the fungus would evolve much more slowly than a virus.

McCallum said that it would be difficult to test which of the hypotheses were right, but if the evolution theory was true it could hold out help for endangered rare frog species.

"If there are frog species surviving with the fungus, we may be able to select these frogs and breed them to create a generation of frogs resistant to the fungus."

He said it would also be interesting to do controlled infections in a non-endangered species using fungus from different sources to test the virulence of the disease.

In Australia, 27 species of frog are currently endangered or threatened and four have become extinct.